March 6, 2012 -- There's good news for women with hysterectomies who are considering taking the hormone estrogen to ease hot flashes and other menopausal complaints.

New results from a long-running government study of the effect of hormones on women's health show that estrogen-only appears to cut a woman's risk of getting breast cancer by about 20% and significantly reduces her risk of dying from the disease. Those benefits appear to last for years after the therapy ends.

"Certainly for some women hormone therapy dramatically improves their quality of life," says Margery Gass, MD, who is the executive director of the North American Menopause Society (NAMS).

NAMS recently updated its position statement on the use of hormone therapy to reflect the new research findings.

In 2002, doctors looking at data from the Women's Health Initiative sounded the alarm about hormone replacement therapy, warning that the risks of heart attacks, strokes, and breast cancers were higher for women taking the combination of estrogen and progestin than for women taking a placebo. Researchers stopped the trial early after noting increased adverse events in women taking hormones.

But experts say that initial reaction to hormone therapy probably wasn't warranted. They say many women can safely take hormones for a short time to quell bad hot flashes, night sweats, and mood swings that can accompany menopause.

"The effects on breast cancer are so different between estrogen and progestin and estrogen alone that we felt it was time to say that women taking estrogen alone may not need to stop at three to five years. Because over that amount of time, no breast cancer showed up in women taking estrogen alone," says Gass, who is also a co-author on the new study.

Estrogen and progestin are typically prescribed together when a woman still has her uterus to reduce the risk of uterine cancer while promoting the benefits of estrogen therapy.

Yet estrogen alone is used in women who've had a hysterectomy, or removal of the uterus. Statistics show about one-third of women in the U.S. have had a hysterectomy by the time they are 60.

Findings 'Clearly Very Reassuring'

For the latest study, which is published in The Lancet Oncology, researchers kept tabs on more than 7,600 women who took part in the estrogen-only treatment arm of the trial.

The women assigned to get estrogen took that hormone for about six years before they stopped. They have now been followed for nearly five years beyond their estrogen use.

Compared to women taking a placebo, women who took estrogen had a 23% reduced risk of invasive breast cancer. That means 151 women got breast cancer in the estrogen group compared to 199 women assigned to the placebo.

Women taking estrogen also had a 63% reduced risk of dying from breast cancer compared to women on the placebo. Overall, there were six deaths in the estrogen group compared to 16 in the placebo group.

In an email, Anthony Howell, MD, a professor of medical oncology at the University of Manchester in the U.K., says the study findings are "clearly very reassuring for women."

"However, they have to be counselled concerning the very small increased risk of deep vein thrombosis [blood clot in a vein] and pulmonary embolism [blockage of blood to the lungs], which is seen with any hormones such as the oral contraceptive pill," says Howell, who wrote a comment on the findings but was not involved in the research.

Breast Cancer Benefit May Not Apply to All

Several other important warnings also apply.

The first is that estrogen did not appear to help some women. Those were women at higher risk for breast cancer because they had a family history or history of having benign breast disease. Estrogen may even increase the risk of breast cancer in women who already have other risk factors.

"These agents should not be used for breast cancer prevention, even though we clearly show a lower risk of breast cancer in these women taking hormones," says Anderson, who is also a member of the Fred Hutchinson Cancer Research Center.

The second important consideration is that the study tested a kind of estrogen called conjugated equine estrogen, which is sold under the brand name Premarin. In recent years, that formulation has become less popular. More women have moved toward using estradiol, which is chemically closer to the body's own estrogen. But researchers say they aren't sure if the two kinds of estrogen work the same way.

"Conjugated equine estrogens are very complex pills," Anderson says. "There are a lot of different estrogen compounds in them. We really don't know what the active agents are. To make a leap to another form is really hard to say."